Focusing systems employed in the prior art macro lenses are well known, including a leading lens displacement system or a front focusing system where the foremost group of lens pieces closest to an object being photographed are to be moved for focusing, and an inner focusing system where some other lens group(s) other than the foremost lens group are to be moved for focusing.
The leading lens displacement system, namely, the front focusing system forces the largest and thus heavy lens group located the closest to the object being photographed to move during focusing. Thus, in a camera lens that is adapted to serve as a front focusing macro lens in combination with an automatic focusing mechanism, lens drive power for automatic focusing should be restricted, resulting in the front focusing macro lens encountering a difficulty in quick focusing.
In contrast, the inner focusing system, which forces a relatively lightweight lens group(s) other than the foremost lens group to move during focusing, is suitable to effecting the quick focusing. With the inner focusing system, however, unlike the leading lens displacement system or the front focusing system, the lens optics experiences a greater variation in aberrations in association with a varied object distance, especially, in spherical aberration. This kind of adverse effect tends to be more conspicuous for camera lenses of a greater aperture ratio.
The aforementioned is a primary reason why the prior art inner focusing macro lenses have an F-number as high as 2.8 or even greater. For instance, one exemplary macro lens capable of equi-magnification image focusing as disclosed in Document 1 in the following list of cited documents of the related art is a photographing lens of four groups of lens pieces in which a distance between principal points of the first and second lens groups is negative, and during focusing, the first lens group is static while the second lens group is moved toward the imaging plane, and the third lens group is moved toward the object so as to conduct themselves in a manner of so called ‘floating’.
Such a macro lens is typically 50.0 to 60.0 mm in focal length, and 2.8 to 2.9 in F-number.
Another macro lens capable of equi-magnification image focusing as disclosed in Document 2 is a photographing lens of four groups of lens pieces, namely, the 1st lens group of positive power, the 2nd lens group of negative power, the 3rd lens group of positive power, and the 4th lens group of negative power in sequence from a position closest to the object being photographed, in which during focusing from infinity to proximity, the 1st lens group is static while the 2nd lens group is moved toward the imaging plane, and the 3rd lens group is moved toward the object so as to simultaneously meet requirements defined in the formulae as follows:0.58<sk/f<0.700.65<f34/f<0.72where sk is the shortest back focus from infinity to proximity, f is a focal length of the entire optics, and f34 is a composite focal length of the 3rd and 4th lens groups. The focal length is 62.4 mm, and the F-number is 2.9.
A macro lens capable of equi-magnification image focusing as disclosed in Document 3 includes a tele-photographing macro lens optics of four groups of lens pieces, namely, the 1st lens group of positive power, the 2nd lens group of negative power, the 3rd lens group of positive power, and the 4th lens group of negative power in sequence from a position closest to the object being photographed, in which during focusing from infinity to proximity, the 2nd lens group is moved toward the imaging plane while the 3rd lens group is moved toward the object so as to meet requirements defined in the formulae as follows:3.7<|f×m/ΔX3|<9.9where f is a focal length of the entire optics during photographing at infinity, m is a lateral magnification during photographing at the shortest object distance with a magnification of up to 1:1, ΔX3 is a total displacement of the 3rd lens group for focusing from infinity to proximity. The focal length is 200 mm, and the F-number is 4.0.